[Kitahara]

Hhmm, from my engineering experience, I have two suggestions.

Route A: Use a reasonably stiff (not overly stiff) shock, and if you are being very specific and want to go a bit further, spec out a shock that has an exponential spring rate.  Contact some mechanical/engineering suppliers - not too too expensive.  mount that with direct hardware of course.

Route B: Use hard rubber or polyurethane mount inserts if you can construct OR find some that would fit well.  Use a steel or titanium alloy bar and weld it to the outer mount with the polyurethane or hard rubber mount inserts on the interior.  Use steel or titanium, pref titanium, because it flexes a little, atleast far more than say, aluminum.

Route C: Do both, no doubt it will be better, but would it be worth it or overdoing it.